In radio communication systems employing Orthogonal Frequency Division Multiplexing (OFDM systems), a base station transmits symbols to one or more remote units. Each symbol includes one or more packets, and different packets within a symbol may be intended for different remote units. Each packet includes a packet serial number, a receiver address, a header checksum, a payload, and a packet checksum. The packet serial number and the receiver address form a header. When a remote unit receives a symbol, it decodes the header of each packet to determine the receiver address of the packet. The remote unit notes packets whose receiver address matches the identity of the remote unit. The remote unit processes the payload of any such packets and passes the data in the payload to a user.
The remote unit performs error detection for a packet at two stages. When the remote unit decodes the header, the remote unit verifies the content of the header using the header checksum of the packet. If the header checksum indicates that the header contains no errors, and if the receiver address matches the identity of the remote unit, then the remote unit decodes the payload and verifies the content of the entire packet using the packet checksum.
Two standard techniques for error correction are Automatic Repeat Request (ARQ) and soft symbol combining. ARQ is a technique by which the remote unit requests retransmission of a packet. Soft symbol combining is a technique by which an entire symbol is retransmitted and the remote unit combines two samples of a symbol, the originally transmitted symbol and the retransmitted symbol, before decoding the symbol in order to reduce the likelihood of errors.
Current OFDM systems do not use ARQ, because OFDM is used primarily for television broadcasts which can not tolerate the delays arising from retransmission. However systems using OFDM for internet traffic can tolerate such delays. Error correction would be improved if the remote unit could combine ARQ with soft symbol combining. Unfortunately, in order to carry out soft symbol combining and take advantage of increased coding benefits, the smallest unit that can be retransmitted is a symbol. If there is only one packet for which retransmission is required, for example if the symbol contains packets for more than one remote unit and only one remote unit detected an error, then retransmission of the entire symbol is inefficient and a waste of bandwidth. Furthermore, retransmission of the symbol may lead to confusion in remote units which were able to decode their packets correctly.